Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 943 



the casing head of an oil well. Reflections are recorded from any kind of 

 obstacle in the well, that is, not only from the oil level, but also from 

 tubing catchers, liner tops, and tubing collars. These echos are picked 

 up by a microphone and are recorded oscillographically on rapidly moving 

 film. Because of the variation of the sound velocity with the nature and 

 temperature of gas admixtures in the well, the lesser reflections from tube 

 collars of known depth-interval are used as a means of calibrating the 

 time scale when necessary. 



8. Noise prevention. Noise prevention or noise reduction becomes in- 

 creasingly important with the growth in the use of industrial machinery. 

 In respect to vehicle traffic, noise prevention methods extend to the reduc- 

 tion of sounds and noises from horns, engines, exhausts, automobile tires, 

 and tram wheels; in construction work they involve decreasing the noise 

 of riveting, and the like; in mechanical and electrical processing, these 

 methods aim at sound-proofing and vibration-insulation of foundations. 

 One important phase of this work is the reduction of reverberations in 

 oflSces and auditoriums and the improvement in their acoustics generally 

 speaking; another phase, of military significance, is the design of airplane, 

 submarine, and battleship engines, shafts, and propellers in such a manner 

 that noise and noise transmission is reduced as much as possible. 



B. Marine-Acoustic Methods 



1. Velocity and absorption of sound in water. Compared with air, water 

 is a much more suitable medium for the transmission of sound. Its 

 velocity is 4^ times as great, its absorption more than thousand times less. 

 Like the velocity of atmospheric sound, the velocity of sounds in a liquid 

 changes with temperature. In marine transmission, significant varia- 

 tions occur with changes in salinity. Following are two useful relations 

 expressing the velocity in water as a function of temperature t (in degrees 

 C.) and salinity u in permille (at 0" C.) : 



(1) Metric (Maurer) formula: 

 Vm.sec-i = 1445 + 4.46t - 0.0615t' + (1.2 - 0.015t)(u - 35) 



(2) English formula : 

 Vft..Bec-i = 4626 + 13.8t - 0.12t' + 3.73u. 



> (12-21) 



Unlike sound in air, sound in water is not much affected by the move- 

 ment of the transmission medium; in other words, oceanic, tidal, and 

 similar currents are ineffective. However, as in air, refraction and reflec- 

 tion occur because of vertical variations in salinity and temperature. The 



